U.S. patent application number 10/721783 was filed with the patent office on 2004-08-19 for polarizing electrode for electric double layer capacitor and electric double layer capacitor therewith.
Invention is credited to Iwaida, Manabu, Murakami, Kenichi, Oki, Naohiko, Otsuka, Kiyoto, Oyama, Shigeki, Ozaki, Kouki, Tsutsui, Masanori.
Application Number | 20040160728 10/721783 |
Document ID | / |
Family ID | 32854410 |
Filed Date | 2004-08-19 |
United States Patent
Application |
20040160728 |
Kind Code |
A1 |
Oyama, Shigeki ; et
al. |
August 19, 2004 |
Polarizing electrode for electric double layer capacitor and
electric double layer capacitor therewith
Abstract
A polarizing electrode for an electric double layer capacitor
has good moldability and can achieving higher density of electrode
and higher capacity, and an electric double layer capacitor employs
the same. The electric double layer capacitor is made of an
activated carbon obtained by activating a hard-to-graphitize
material (for example, phenol resin) with water vapor, and the
activated carbon has a median particle size within a range from 4
.mu.m to 8 .mu.m in the particle size distribution and at least a
peak observed on the side of smaller particle size than the median
particle size in the particle size distribution.
Inventors: |
Oyama, Shigeki; (Shioya-gun,
JP) ; Iwaida, Manabu; (Saitama-shi, JP) ; Oki,
Naohiko; (Oyama-shi, JP) ; Murakami, Kenichi;
(Utsunomiya-shi, JP) ; Ozaki, Kouki; (Kasugai-shi,
JP) ; Tsutsui, Masanori; (Kuwana-shi, JP) ;
Otsuka, Kiyoto; (Okayama-shi, JP) |
Correspondence
Address: |
ARENT FOX KINTNER PLOTKIN & KAHN
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Family ID: |
32854410 |
Appl. No.: |
10/721783 |
Filed: |
November 26, 2003 |
Current U.S.
Class: |
361/502 ;
423/449.1 |
Current CPC
Class: |
Y02E 60/13 20130101;
H01M 4/583 20130101; H01G 11/24 20130101; H01M 2004/021 20130101;
H01G 11/34 20130101; C01B 32/30 20170801; C01B 32/336 20170801;
H01G 11/42 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
361/502 ;
423/449.1 |
International
Class: |
H01G 009/155; H01G
009/042; C01B 031/08 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2002 |
JP |
2002-349170 |
Nov 29, 2002 |
JP |
2002-349172 |
Oct 29, 2003 |
JP |
2003-369379 |
Oct 29, 2003 |
JP |
2003-369381 |
Claims
What is claimed is:
1. Activated carbon, obtained by activating a hard-to-graphitize
material with water vapor, has a median particle size ranging a
range from 4 .mu.m to 8 .mu.m in a particle size distribution when
measured by laser diffraction method, and the particle size
distribution has at least a peak located at a particle size which
is lower than the median particle size.
2. Activated carbon according to claim 1, wherein activated carbon
particles of not larger than 2 .mu.m is not less than 10% by weight
in a cumultive distribution of the activated carbon particles.
3. A polarizing electrode for an electric double layer capacitor,
comprising an activated carbon obtained by activating a
hard-to-graphitize material with water vapor, wherein the activated
carbon has a median particle size within a range from 4 .mu.m to 8
.mu.m in the particle size distribution as measured by a laser
diffraction method and has at least a peak observed on the side of
smaller particle size than the median particle size in the particle
size distribution.
4. The polarizing electrode for an electric double layer capacitor
according to claim 1, wherein the activated carbon contains 10% or
more in accumulated percentage of particles having sizes not larger
than 2 .mu.m.
5. An electric double layer capacitor comprising an electrode unit
comprising a current collector and polarizing electrode, a
separator and an electrolytic solution, wherein the polarizing
electrode is made of an activated carbon obtained by activating a
hard-to-graphitize material with water vapor, while the activated
carbon has a median particle size within a range from 4 .mu.m to 8
.mu.m in the particle size distribution as measured by a laser
diffraction method and has at least a peak observed on the side of
smaller particle size than the median particle size in the particle
size distribution.
6. Activated carbon, obtained by activating a hard-to-graphitize
material with water vapor, wherein the activated carbon particles
comprises not less than 10% by weight of particles not larger than
2 .mu.m in a cumultive distribution and particles which bulk
density is within a range of 0.18 g/cm.sup.3 to 0.25
g/cm.sup.3.
7. Activated carbon according to claim 6, wherein a fluidity index
of the activated carbon particles is within a range of 0.47 to
0.52.
8. A polarizing electrode for an electric double layer capacitor,
comprising an activated carbon obtained by activating a
hard-to-graphitize material with water vapor, wherein the activated
carbon contains 10% or more in accumulated percentage of particles
having sizes not larger than 2 .mu.m and has a bulk density within
a range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3.
9. The polarizing electrode for an electric double layer capacitor
according to claim 8, wherein the activated carbon has a fluidity
index within a range from 0.47 to 0.52.
10. An electric double layer capacitor comprising an electrode unit
comprising a current collector and polarizing electrode, a
separator and an electrolytic solution, wherein the polarizing
electrode is made of an activated carbon obtained by activating a
hard-to-graphitize material with water vapor, and the activated
carbon contains 10% or more in accumulated percentage of particles
having sizes not larger than 2 .mu.m and has a bulk density within
a range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3.
Description
BACKGROUND OF THE INVENTION
[0001] Priority is claimed on Japanese Patent Application No.
2002-376504, filed Dec. 26, 2003, the content of which is
incorporated herein by reference.
[0002] 1. Field of the Invention
[0003] The present invention relates to a polarizing electrode for
an electric double layer capacitor and to an electric double layer
capacitor using the polarizing electrode.
[0004] 2. Description of Related Art
[0005] An electric double layer capacitor utilizes electrical
energy stored in an electric double layer which is formed at the
interface between a polarizing electrode and an electrolytic
solution.
[0006] The electric double layer capacitor has a large Farad level
capacity and excellent charge and discharge cycle characteristic,
and it is therefore used in applications such as backup power
sources for electrical equipment and vehicle-mounted batteries.
[0007] Referring to FIG. 8, for example, an electric double layer
capacitor 1 has two polarizing electrodes incorporated therein,
namely, a first electrode 2 and a second electrode 3. The first
electrode 2 and the second electrode 3 are separated from each
other by a separator 4.
[0008] The first electrode 2 and a first current collector
(hereinafter also referred to as a cap) 5 that is disposed outside
the former constitutes one electrode unit 7 and functions as an
anode. The second electrode 3 and a second current collector
(hereinafter also referred to as a casing) 6 that is disposed
outside the former constitute another electrode unit 8 that
functions as a cathode. Activated carbon that has microscopic pores
is preferably used for the first electrode 2 and the second
electrode 3 that constitute the electric double layer capacitor 1
(Japanese Patent Application, First Publication No. Hei
9-320906).
[0009] As shown in FIG. 9, the two polarizing electrodes 11, 12
made of activated carbon that constitute the electric double layer
capacitor are impregnated with an electrolytic solution 15
consisting of a solvent and an electrolyte. Electrolyte ions 16 and
17 are adsorbed so as to congregate in the pores 18 and 19 of the
activated carbon that forms the two polarizing electrodes 11 and 12
through salvation with the electrolytic solution 15, so that the
polarizing electrode 11 and the electrode unit 13 form an anode
while the other polarizing electrode 12 and the electrode unit 14
form a cathode.
[0010] The activated carbon that makes the two electrodes can be
regarded as providing a place for the solvent and the electrolyte
ions to act electrochemically with each other thereon. Thus the
physical properties and microscopic structure of the activated
carbon are among the factors that have great influence on the
performance of the electric double layer capacitor.
[0011] As another example of the electric double layer capacitor
described above, such a capacitor is known as an electrode unit
formed in a sheet with a metallic body having electrical
conductivity made of foil (hereinafter referred to as an
electrically conductive metal foil) pasted using an electrically
conductive adhesive into an integral member which is wound. For the
electrically conductive metal foil, for example, a foil made of a
metal such as aluminum (Al) is preferably used as it is after being
etched on the surface thereof.
[0012] One of characteristics required of an electrode of a
capacitor of high output power (about 250 W per cell) intended for
use in an automobile is a low internal resistance and sufficient
capacity that enables it to draw a large current.
[0013] Capacity of a capacitor can be increased by increasing the
capacity per unit weight of the electrode (F/g) In the case in
which there is a limitation to the volume of the capacitor module
for installation in an automobile or the like, capacity per unit
volume of electrode (F/cc) must be increased instead of capacity
per unit weight of electrode (F/g). Increasing the capacity per
unit volume of electrode (F/cc) means increasing the molding
density of the electrode.
[0014] For increasing the molding density of electrode, such
methods are known as increasing the density of activated carbon
without decreasing the capacity per unit weight, or molding the
electrode in a close-packed structure.
[0015] As the former method, that is, to increase the density of
activated carbon, Japanese Patent Application, First Publication
No. Hei 9-320906 discloses such a method in which an easy to
graphitize material is used to make the activated carbon, so that a
carbon material obtained by carbonizing the former at a temperature
of 1000.degree. C. or lower in an inert atmosphere is activated
with a hydroxide of an alkali metal, thereby producing activated
carbon.
[0016] However, since a manufacturing process that employs
activation with a chemical makes it difficult to control the
activation process and requires a process of washing off the
chemical to such a level that it does not affect the operation of
the capacitor after the activation, many problems remain to be
solved from the viewpoint of manufacturing cost, before the process
can be employed for mass production.
[0017] For stable production of activated carbon, it is known to
activate carbon with a gas such as water vapor, instead of the
chemical. In this case, carbon made by carbonizing a
hard-to-graphitize material at a temperature around 1000.degree. C.
in an inert atmosphere is used. In the case of this method, there
has been a problem in that since a hard-to-graphitize material
which is relatively easy to activate is used, formation of
microscopic pores in the activated carbon proceeds excessively and,
as a result, density of the activated carbon tends to decrease.
[0018] As methods of the latter category, that is, to form the
electrode having a close-packed structure, there are methods such
as one in which the density of an electrode sheet is increased by
controlling the load of rolling when forming the electrode sheet
(Japanese Patent Application, First Publication No. 2000-277391),
and a method of controlling the particle size of the activated
carbon that is the main component (Japanese Patent Application,
First Publication No. 2001-52972).
[0019] However, an electrode formed to have a high density by any
of the methods described above has problems such as cracking,
rupture or other significant molding defect occurring in the molded
sheet, or problems such as a decrease in the infiltration rate of
the electrolytic solution or insufficient impregnation occurring in
the process of impregnating with the electrolytic solution during
assembly of the capacitor.
[0020] There has also been a problem in that it is difficult to
determine whether the activated carbon is good as a stock feed in
advance, since the molding characteristic and the electrode density
can be evaluated only after the material is formed into a
sheet.
[0021] [Reference 1] Japanese Patent Application, First Publication
No. Hei 9-320906
[0022] [Reference 2] Japanese Patent Application, First Publication
No. 2000-277391
[0023] [Reference 3] Japanese Patent Application, First Publication
No. 2001-52972
BRIEF SUMMARY OF THE INVENTION
[0024] In view of the background described above, an object of the
present invention is to provide a polarizing electrode for an
electric double layer capacitor which has better moldability and
allows it to increase both the density and capacity of the
electrode, and an electric double layer capacitor using the
same.
[0025] To achieve the above object, the present invention has been
studied through two approaches as represented by a first aspect and
a second Aspect.
[0026] First Aspect
[0027] The first aspect of the present invention provides activated
carbon, obtained by activating a hard-to -graphitize material (for
example, phenol resin) with water vapor, has a median particle size
ranging a range from 4 .mu.m to 8 .mu.m in a particle size
distribution when measured by laser diffraction method, and the
particle size distribution has at least a peak located at a
particle size which is lower than the median particle size.
[0028] In the activated carbon shown above, the activated carbon
particles of less than 2 .mu.m exceeds 10% by weight in the
particle size distribution of the activated carbon particles.
[0029] The first aspect of the present invention also provides the
polarizing electrode for electric double layer capacitor,
comprising an activated carbon obtained by activating
hard-to-graphitize material with water vapor, wherein the activated
carbon has a median particle size within a range from 4 .mu.m to 8
.mu.m in the particle size distribution as measured by a laser
diffraction method and has at least a peak observed on the side of
smaller particle size than the median particle size in the particle
size distribution.
[0030] The activated carbon having the particle size distribution
observed by a laser diffraction method (using, for example,
SALD-3000S analyzer of Shimadzu Corporation, described in detail in
the embodiment) and different median particle size can be obtained
on a stable basis, by activating the hard-to-graphitize material
(for example, phenol resin, described in the embodiment) with water
vapor.
[0031] In the case in which the activated carbon has a median
particle size smaller than 4 .mu.m in the particle size
distribution, the strength of the electrode sheet decreases
monotonically as the median particle size decreases. In the case in
which the median particle size is larger than 4 .mu.and less than 8
.mu.m, on the other hand, the strength of the electrode sheet
decreases sharply as the median particle size increases. Activated
carbon having median particle size within a range from 4 .mu.to 8
.mu.in the particle size distribution is preferable since it
enables it to ensure very high strength of the electrode sheet
around 5 kgf/cm.sup.2.
[0032] The activated carbon having at least a peak observed on the
side of smaller particle size than the median particle size in the
particle size distribution is preferable since it enables it to
ensure very high strength of the electrode sheet around 5
kgf/cm.sup.2 while such problems as cracking, rupture or other
significant molding defects do not occur in the electrode sheet.
The activated carbon having such a particle size distribution as
described above also enables it to achieve a relatively high
density of the electrode sheet above 0.630 g/cm.sup.3.
[0033] The present invention also provides the polarizing electrode
for electric double layer capacitor wherein the activated carbon
contains 10% or more particles having sizes not larger than 2 .mu.m
in terms of accumulated percentage.
[0034] The activated carbon contains 10% or more particles of sizes
not larger than 2 .mu.m in terms of accumulated percentage is also
preferable since it enables it to ensure very high strength of the
electrode sheet around 5 kgf/cm.sup.2, while such problems as
cracking, rupture or other significant molding defects do not occur
in the electrode sheet. This activated carbon can also make the
electrode sheet that has relatively high density of above 0.630
g/cm.sup.3. It was confirmed that, in the case of activated carbon
that contains less than 10% particles having sizes not larger than
2 .mu.m in terms of accumulated percentage, the strength of the
electrode sheet shows a tendency to decrease, resulting in poor
molding characteristic, and density of the electrode sheet also
shows a tendency to decrease.
[0035] Based on the results described above, a polarizing electrode
for an electric double layer capacitor using the activated carbon
that has a median particle size within a range from 4 .mu.m to 8
.mu.m in the particle size distribution and has at least a peak
observed on the side of smaller particle size than the median
particle size in the particle size distribution, and contains 10%
or more particles having sizes of not larger than 2 .mu.m in terms
of accumulated percentage has both relatively high strength and
density of the electrode sheet, and therefore can be better molded,
and enables it to increase the density and capacity of the
electrode. As a result, stability of operation for handling the
polarizing electrode is improved, and therefore manufacturing cost
can be decreased. The high density of the electrode sheet also
contributes to the manufacture of a highly dense polarizing
electrode.
[0036] Second Aspect
[0037] The second aspect of the present invention also provides an
electric double layer capacitor comprising an electrode unit
comprising a current collector and a polarizing electrode, a
separator and an electrolytic solution, wherein the polarizing
electrode is made of an activated carbon obtained by activating a
hard-to-graphitize material with water vapor, and the activated
carbon has a median particle size within a range from 4 .mu.m to 8
.mu.m in the particle size distribution as measured by a laser
diffraction method and has at least a peak observed on the side of
smaller particle size than the median particle size in the particle
size distribution.
[0038] With such a constitution, since the activated carbon that
makes the polarizing electrode of the electric double layer
capacitor has a median particle size in a range described above
(from 4 .mu.m to 8 .mu.m) and the particle size distribution
(having at least a peak observed on the side of smaller particle
size than the median particle size) and shows the ratio of capacity
maintained after 2000 hours of around 90%, it is possible to
provide an electric double layer capacitor that exhibits high
reliability over a long period of time.
[0039] Second Aspect
[0040] Activated carbon, obtained by activating a
hard-to-graphitize material with water vapor, wherein the activated
graphite particles comprises more than 10% by weight of particles
less than 2 .mu.m in a cumultive distribution and particles which
bulk density is within a range of 0.18 g/cm.sup.3 to 0.25
g/cm.sup.3
[0041] In the activated carbon described above, a fluidity index of
the activated carbon particles is within a range of 0.47 to
0.52.
[0042] Furthermore, the polarizing electrode for an electric double
layer capacitor using the above-described activated carbon has a
median particle size within a range from 4 .mu.m to 8 .mu.m in the
particle size distribution and has at least a peak observed on the
side of smaller particle size than the median particle size in the
particle size distribution, and is made by using activated carbon
that contains 10% or more particles having sizes not larger than 2
.mu.m in terms of accumulated percentage, and therefore it is made
possible to have both relatively high strength and density of the
electrode sheet that lead to good molding characteristics of the
polarizing electrode.
[0043] The good molding characteristics of the polarizing electrode
described above improves stability of operation for handling the
polarizing electrode and therefore decreases the manufacturing
cost, and high density of the electrode sheet contributes to the
manufacture of highly dense polarizing electrodes.
[0044] The electric double layer capacitor according to the present
invention can achieve a high ratio of capacity maintained after
2000 hours of around 90%, since the polarizing electrode of the
constitution described above is used.
[0045] As a result, the present invention can provide a polarizing
electrode for an electric double layer capacitor and an electric
double layer capacitor that allow decreased cost and have high
performance and long-term reliability.
[0046] The second aspect of the present invention, in order to
achieve the above object, provides a polarizing electrode for an
electric double layer capacitor comprising an activated carbon
obtained by activating a hard-to-graphitize material with water
vapor, wherein the activated carbon contains 10% or more of
particles having sizes not larger than 2 .mu.m in terms of
accumulated percentage and bulk density within a range from 0.18
g/cm3 to 0.25 g/cm.sup.3.
[0047] By using the activated carbon obtained by activating the
hard-to-graphitize material (for example, phenol resin described in
the embodiment) with water vapor, those having the content of
activated carbon particles not greater than 2 .mu.m, bulk density
(measured, for example, by bulk density measuring method for
particulate activated carbon specified in JIS K1474 to be described
in detail in the embodiment) and fluidity index (measured, for
example, by a weight tap density measuring method using Tap Denser
KYT-3000 manufactured by SEISHIN ENTERPRISE CO., LTD. to be
described in detail in the embodiment) having different values can
be obtained on a stable basis.
[0048] Activated carbon that contains 10% or more particles of
sizes not larger than 2 .mu.m in terms of accumulated percentage is
preferable since it enables it to ensure very high strength of the
electrode sheet of around 5 kgf/cm.sup.2 while such problems as
cracking, rupture or other molding defects do not occur in the
electrode sheet. The activated carbon having a particle size
distribution such as described above can also yield on electrode
sheet that has relatively high density of above 0.630 g/cm.sup.3.
It was confirmed that, in the case of activated carbon having 10%
or more particles having sizes not larger than 2 .mu.m in terms of
accumulated percentage, the strength of the electrode sheet shows a
tendency to decrease, trend resulting in poor molding
characteristics, and the density of the electrode sheet also shows
a tendency to decrease.
[0049] With activated carbon that has a bulk density less than 0.18
g/cm.sup.3 or higher than 0.25 g/cm.sup.3, relatively high density
of the electrode sheet above 0.630 g/cm.sup.3 cannot be achieved on
a stable basis and the strength of the electrode sheet shows a
substantial decrease. Activated carbon that has a bulk density
within a range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3, in
contrast, is preferable, since it causes the density of the
electrode sheet to increase monotonically at relatively high levels
above 0.630 g/cm.sup.3 as the bulk density increases within this
range, and very high strength of the electrode sheet around 5
kgf/cm.sup.2 can be achieved.
[0050] The second aspect of the present invention also provides the
polarizing electrode for electric double layer capacitor wherein
the activated carbon has a fluidity index within a range from 0.47
to 0.52 as calculated by Kawakita's formula from the tap
density.
[0051] When the fluidity index of the activated carbon is below
0.47 or over 0.52, relatively high density of the electrode sheet
above 0.630 g/cm.sup.3 cannot be obtained on a stable basis, and
the strength of the electrode sheet shows a substantial decrease.
When the activated carbon has a fluidity index within a range from
0.47 to 0.52, in contrast, it was confirmed that density of the
electrode sheet decreases monotonically at relatively high levels
above 0.630 g/cm.sup.3 as the fluidity index increases within this
range, and very high strength of the electrode sheet around 5
kgf/cm.sup.2 can be achieved.
[0052] Based on the results described above, a polarizing electrode
for an electric double layer capacitor using the activated carbon
that contains 10% or more particles having sizes not larger than 2
.mu.m in terms of accumulated percentage, a bulk density within a
range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3 and a fluidity index
within a range from 0.47 to 0.52 has both relatively high strength
and density of the electrode sheet, and therefore can be better
molded and enables it to increase the density and capacity of the
electrode. As a result, stability of operation for handling the
polarizing electrode is improved, and therefore manufacturing cost
can be decreased. The high density of the electrode sheet also
contributes to the manufacture of highly dense polarizing
electrodes.
[0053] The present invention also provides an electric double layer
capacitor comprising an electrode unit comprising a current
collector and a polarizing electrode, a separator and an
electrolytic solution, wherein the polarizing electrode is made of
an activated carbon obtained by activating a hard-to-graphitize
material with water vapor, and the activated carbon contains 10% or
more particles having sizes not larger than 2 .mu.m in terms of
accumulated percentage and bulk density within a range from 0.18
g/cm.sup.3 to 0.25 g/cm.sup.3.
[0054] With such a constitution, since the activated carbon that
makes the polarizing electrode of the electric double layer
capacitor contains 10% or more particles having sizes not larger
than 2 .mu.m in terms of accumulated percentage, bulk density
within a range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3 and can
maintain the ratio of capacity maintained after 2000 hours around
90%, it is made possible to provide the electric double layer
capacitor that demonstrates high reliability over a long period of
time.
[0055] The polarizing electrode for an electric double layer
capacitor according to the second aspect of the present invention
is manufactured by using the activated carbon containing 10% or
more particles having sizes not larger than 2 .mu.m in terms of
accumulated percentage, a bulk density within a range from 0.18
g/cm.sup.3 to 0.25 g/cm.sup.3 and a fluidity index within a range
from 0.47 to 0.52, and therefore has both relatively high strength
and density of the electrode sheet, that leads to good molding
characteristics of the polarizing electrode.
[0056] The good molding characteristics of the polarizing electrode
described above improves stability of operation for handling the
polarizing electrode and therefore decreases the manufacturing
cost, and high density of the electrode sheet contributes to the
manufacture of highly dense polarizing electrode.
[0057] The electric double layer capacitor according to the present
invention can achieve a high ratio of capacity maintained after
2000 hours of around 90%, by using the polarizing electrode of the
constitution described above.
[0058] As a result, the present invention can provide a polarizing
electrode for an electric double layer capacitor and an electric
double layer capacitor that allows it to decrease the cost and have
high performance and long-term reliability.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0059] FIG. 1 is a graph showing particle size distribution of the
activated carbon obtained in Example 2.
[0060] FIG. 2 is a graph showing particle size distribution of the
activated carbon obtained in Example 4.
[0061] FIG. 3 is a graph showing particle size distribution of the
activated carbon obtained in Example 5.
[0062] FIG. 4 is a graph showing particle size distribution of the
activated carbon obtained in Comparative Example 1.
[0063] FIGS. 5A and 5B are graphs showing the relationship between
the proportion of particles having median particle size (5A) and
the relationship between a content of fine particles not exceeding
2 .mu.m and density of electrode sheet (5B).
[0064] FIGS. 6A and 6B are graphs showing the relation between the
proportion of particles having median particle size (6A) and the
content of fine particles not exceeding 2 .mu.m and density of
electrode sheet (6B).
[0065] FIGS. 7A and 7B are graphs showing the accumulated
percentages of particle size distribution in Example 2 and
Comparative Example 4 (7A) and the accumulated percentages of
particle size distribution in Example 2 and Comparative Example 2
(7B) calculated from the measurements of particle size distribution
of the activated carbon in Examples and Comparative Examples.
[0066] FIG. 8 shows an example of an electric double layer
capacitor.
[0067] FIG. 9 is a schematic diagram showing ions contained in the
electrolytic solution being adsorbed by the electrode.
[0068] FIG. 10 is a graph showing the result of a Kawakita's tap
density measuring method.
[0069] FIGS. 11A and 11B are graphs showing the relation between
the bulk density (11A) or fluidity index (11B) of the activated
carbon and density of the electrode sheet.
[0070] FIGS. 12A and 12B are graphs showing the relation between
the bulk density (12A) or fluidity index (12B) of the activated
carbon and the strength of the electrode sheet.
[0071] FIGS. 13A and 13B are graphs showing the accumulated
percentage of particle size distribution in Example 2 and
Comparative Example 4 (13A) and the percentage of the particle size
distribution of Example 2 and Comparative Example 2 (13B)calculated
from the measurements of particle size distribution of the
activated carbon in the Examples and Comparative Examples.
DETAILED DESCRIPTION OF THE INVENTION
[0072] First Aspect of the Present Invention
[0073] The activated carbon that forms the polarizing electrode for
an electric double layer capacitor of the first and second aspect
of the present invention is made by activating a hard-to-graphitize
material with water vapor.
[0074] The term "hard-to-graphitize material" is used for
comprehensive reference to materials made of organic compounds
which are difficult to graphitize. Difficulty in graphitizing means
that it is difficult to form graphite structure by firing at a
temperature of 3000.degree. C. or higher. Formation of graphite
structure can be verified by observing a distinct peak of 2.theta.
around 25.degree. in the X-ray diffraction pattern.
[0075] The activated carbon that constitutes the polarizing
electrode for electric double layer capacitor of the first aspects
of the present invention is preferably made in a manufacturing
process based on the method described below.
[0076] First, the stock feed used to manufacture the activated
carbon of the present invention will be described. A
hard-to-graphitize material which is difficult to graphitize is
preferably used as the stock feed to manufacture the activated
carbon of the present invention. A graphitizing catalyst may be
added during graphitization. As the organic compounds which are
difficult to graphitize, aromatic compounds such as furfuryl
alcohol, polycarbonate, cellulose and phenol resin, and aliphatic
compounds such as epoxy resin, PVDF (polyvinylidene fluoride),
polyvinyl alcohol, nylon and polypropylene may be used.
[0077] The activated carbon that constitutes the polarizing
electrode for an electric double layer capacitor of the first
aspects of the present invention can be manufactured in a procedure
described below by using the materials described above. A process
of using phenol resin as the hard-to-graphitize material which is
fired at such a temperature as a graphite structure is formed and
is then activated with water vapor will herein be described.
[0078] The heat treatment at a temperature so as to form a graphite
structure is carried out at a temperature usually within a range
from 400 to 1000.degree. C., preferably from 500 to 800.degree. C.,
and more preferably from 500 to 700.degree. C., in a non-oxidizing
atmosphere, for example, in the presence of nitrogen (N.sub.2) gas
flow. Duration of the treatment is normally up to 24 hours,
preferably from 1 to 10 hours, and more preferably from 2 to 5
hours. Other conditions for the treatment may be determined in
accordance to such factors as the material to be used and the kind
of electrode to be made.
[0079] Activation with water vapor can be carried out by an
ordinary method. In a preferred embodiment, activation with water
vapor is carried out as follows. A scrubbing bottle containing pure
water is kept at a temperature within a range from room temperature
to 100.degree. C., preferably at 80.degree. C., and nitrogen gas is
passed through the water and activation is carried out by means of
the nitrogen gas containing water vapor. Specifically, temperature
is raised to a level from 800 to 1000.degree. C., preferably
900.degree. C. under the presence of nitrogen gas flow and, after a
predetermined temperature (for example, 800.degree. C.) has been
reached, activation is carried out by using a mixed gas of nitrogen
and water vapor for a period from 5 minutes to 10 hours.
[0080] The activated carbon prepared as described above is crushed
by a jet mill, ball mill or the like for 24 to 300 hours, and the
crushed powder is classified with a sieve of 330 mesh (45 .mu.m)
and thereby activated carbon having predetermined particle size
distribution for the polarizing electrode for electric double layer
capacitor of the present invention is obtained.
[0081] For the activated carbon prepared as described above,
activated carbon that has a median particle size within a range
from 4 .mu.m to 8 .mu.m in the particle size distribution observed
by a laser diffraction method and at least a peak observed on the
side of smaller particle size than the median particle size in the
particle size distribution, and contains 10% or more particles
having sizes not larger than 2 .mu.m in terms of accumulated
percentage can be made.
[0082] It was confirmed by the measuring method specified in JIS
standard K1474-1991, that the amount of benzene adsorbed by the
activated carbon thus obtained was not less than 54% and not more
than 60% by weight of benzene.
[0083] The amount of benzene adsorption is an index that represents
the degree of activation which is determined by measuring the
weight difference of benzene vapor adsorbed onto the activated
carbon.
[0084] The activated carbon can then be used to make the polarizing
electrode for an electric double layer capacitor by an ordinary
method. In order to make a sheet-like polarizing electrode, for
example, the following method may be used.
[0085] The activated carbon made from phenol resin, graphite powder
used as an electrically conductive filler and ethylene
polytetrafluoride used as a binder are mixed in predetermined
proportions (for example, 90:5:5 in weight proportion) and rolled
into a sheet 150 .mu.m in thickness. The sheet is punched through
in a circular shape to make a polarizing electrode having a
diameter of 20 mm.
[0086] Then, as shown in FIG. 8, an electrode unit made by
sandwiching a separator by two sheet-like polarizing electrodes 2
and 3 is disposed in a casing 6 made of an electrically conductive
material that serves as the cathode. After pouring an electrolytic
solution into the casing, a cap 5 made of an electrically
conductive material that serves as the anode is placed thereon and
edges of the casing 6 and the cap 5 are calked together via a
packing 9 made of an insulating material for sealing, thereby
making the electric double layer capacitor 1.
[0087] The median particle size, which is an index that represents
the activated carbon, can be determined from particle size
distribution observed by a laser diffraction method (using, for
example, SALD-3000S analyzer of Shimadzu Corp). From accumulated
frequencies of the particle size distribution, content of activated
carbon particles not larger than 2 .mu.m can be determined.
[0088] The strength of the electrode sheet which is an index that
represents the performance of the capacitor can be determined by
measuring the tensile strength by using, for example, EZ Test-100N
of Shimadzu Corporation. The density of the electrode sheet can be
determined, for example, by measuring the apparent density using a
micrometer.
[0089] The ratio of capacity maintained after endurance is the
electrostatic capacity demonstrated after 2000 hours of 2.5 V
continuous voltage application test at 45.degree. C. divided by the
electrostatic capacity demonstrated before applying the voltage,
given in percentage. The electrostatic capacity can be determined
from the energy discharged by repeating charge and discharge with
predetermined values of voltage and current (for example, charge
voltage of 2.5 V and charge current of 5 mA) using a predetermined
electrolytic solution (for example, propylene carbonate solution of
triethylmethyl ammonium tetrafluoroborate: TEMA.cndot.BF.sub.4/PC,
1.8 mol/l in concentration).
Examples
[0090] The first embodiment of the present invention will be
described below by way of examples, but it should be noted that the
present invention is not limited by the following examples.
Example 1
[0091] The activated carbon powder of this example was made by the
following procedure.
[0092] (1) A phenol resin granulated to have a particle size of
about 3 mm was carbonized by maintaining in a nitrogen gas flow at
900.degree. C. for two hours.
[0093] (2) Carbon thus obtained was heated again in the nitrogen
gas flow and, when the temperature reached 800.degree. C., nitrogen
gas containing 5% of water vapor and 5% of carbon dioxide was
supplied and the carbon was kept at 900.degree. C. (hereinafter
referred to as an activation temperature) for two hours
(hereinafter referred to as an activation time), so as to be
activated.
[0094] (3) The activated carbon thus obtained was left to cool
down, and was crushed into the activated carbon of this example by
using a ball mill employing high-purity alumina balls and operated
at rotational speed of 15 rpm for 150 hours (hereinafter referred
to as a crushing time).
[0095] The median particle size of the activated carbon made as
described above was determined from the particle size distribution
observed by a laser diffraction method (using, for example,
SALD-3000S analyzer manufactured by Shimadzu Corporation).
[0096] Content of activated carbon particles not larger than 2
.mu.m in the activated carbon made as described above was
determined from the accumulated frequency.
[0097] The activated carbon was mixed with 5% by weight of Teflon
7J.RTM. (manufactured by Du Pont-Mitsui Fluorochemicals Co., LTD.)
and 5% of Denka Black.RTM. (manufactured by DENKI KAGAKU KOGYO
KABUSHIKI KAISHA) added thereto as binder, and was formed by
pressure powder molding into a polarizing electrode measuring 20 mm
in diameter and 150 .mu.m in thickness measured with a micrometer.
The polarizing electrode was dried at 150.degree. C. in a vacuum
for four hours, and was weighed to determine the density of the
electrode.
[0098] The strength of the electrode was determined by measuring
the tensile strength by using, for example, EZ Test-100N
manufactured by Shimadzu Corporation.
[0099] Table 1 shows the median particle size of the activated
carbon obtained in Example 1, number of peaks observed on the side
of smaller particle size than the median particle size in the
particle size distribution, content of activated carbon particles
not larger than 2 .mu.m, density of the polarizing electrode, the
strength of the electrode and property (moldability) of the
electrode sheet.
[0100] A PC solution of
1.8M(C.sub.2H.sub.5).sub.3CH.sub.3N.BF.sub.4 was used as the
electrolytic solution, and electrostatic capacity was determined
from the energy discharged by charging for two hours with constant
current and constant voltage conducted at charge voltage of 2.5 V
and charge current of 5 mA. The ratio of capacity deterioration
after endurance was determined by dividing the electrostatic
capacity demonstrated after 2000 hours of 2.5 V continuous voltage
application test at 45.degree. C. by the electrostatic capacity
demonstrated before applying the voltage, given in percentage.
[0101] The result showed the ratio of capacity maintained after
2000 hours was 92%.
Examples 2 to 6
[0102] In these examples of the first embodiment of the present
invention, activated carbon was made in the same manner as in
Example 1, except for setting the crushing time to 125 hours
(Example 2), 100 hours (Example 3), 80 hours (Example 4), 60 hours
(Example 5) and 30 hours (Example 6). The median particle size of
the activated carbon, number of peaks observed on the side of
smaller particle size than the median particle size in the particle
size distribution, content of particles not larger than 2 .mu.m,
density of the polarizing electrode, the strength of the polarizing
electrode and property (moldability) of the electrode sheet were
also determined by similar methods. These figures are also shown in
Table 1.
[0103] The ratio of capacity maintained after 2000 hours was 91%
for the electrode sheet made by using the activated carbon obtained
in Example 2, ratio of capacity maintained after 2000 hours was 90%
in the case of Example 3, ratio of capacity maintained after 2000
hours was 92% in the case of Example 4, ratio of capacity
maintained after 2000 hours was 92% in the case of Example 5 and
ratio of capacity maintained after 2000 hours was 92% in the case
of Example 6.
1TABLE 1 Median Content of Number of peaks particle particles not
not larger than Electrode Electrode Example size larger than 2
.mu.m median particle size in density strength Property No. (.mu.m)
(Weight %) particle size distribution (g/cc) (kgf/cm.sup.2) of
sheet 1 4.2 15.3 3 0.670 5.0 Good 2 4.8 14.3 3 0.662 5.2 Good 3 5.2
12.5 2 0.658 5.1 Good 4 6.5 12.1 2 0.652 5.3 Good 5 7.1 11.3 1
0.644 5.1 Good 6 8.0 10.1 1 0.636 4.9 Good
[0104] FIG. 1 shows the particle size distribution of the activated
carbon obtained in the Example 2, FIG. 2 shows the particle size
distribution of the activated carbon obtained in Example 4, and
FIG. 3 shows the particle size distribution of the activated carbon
obtained in the Example 5. Alternating dotted dash lines in FIGS. 1
to 3 indicate the median particle sizes in the particle size
distributions.
[0105] The amounts of benzene adsorbed by the activated carbon
obtained in Examples 1 to 6 were measured by the measuring method
specified in JIS standard K1474-1991 described above, and it was
confirmed that the amount of benzene adsorbed was not less than 54%
and not more than 60% by weight of benzene.
Comparative Example 1
[0106] In this example, activated carbon was made in the same
manner as in Example 1, except for setting the rotational speed of
the crusher to 25 rpm and crushing time to 40 hours. The median
particle size of the activated carbon, number of peaks observed on
the side of smaller particle size than the median particle size in
the particle size distribution, content of activated carbon
particles not larger than 2 .mu.m, density of the polarizing
electrode, are the strength of the electrode and property
(moldability) of the electrode sheet were also determined by
similar methods. These figures are shown in Table 2.
[0107] The ratio of capacity maintained after 2000 hours was 83%
for the electrode sheet made by using the activated carbon obtained
in Comparative Example 1.
Comparative Examples 2 to 5
[0108] In these examples, activated carbon was made in the same
manner as in Comparative Example 1, except for setting the
rotational speed of the crusher to 25 rpm and crushing time to 45
hours (Comparative Example 2), rotational speed of the crusher to
25 rpm and the crushing time to 35 hours (Comparative Example 3),
rotational speed of the crusher to 35 rpm and the crushing time to
30 hours (Comparative Example 4), and rotational speed of the
crusher to 40 rpm and the crushing time to 30 hours (Comparative
Example 5). The median particle size of the activated carbon,
number of peaks observed on the side of smaller particle size than
the median particle size in the particle size distribution, content
of activated carbon particles not larger than 2 .mu.m, density of
the polarizing electrode, the strength of the electrode and
property (moldability) of the electrode sheet were also determined
by similar methods. These figures are also shown in Table 2.
[0109] The ratio of capacity maintained after 2000 hours was 86%
for the electrode sheet made by using the activated carbon obtained
in Comparative Example 2, ratio of capacity maintained after 2000
hours was 85% in the case of Comparative Example 3, ratio of
capacity maintained after 2000 hours was 84% in the case of
Comparative Example 4, and of capacity maintained after 2000 hours
was 86% in the of Comparative Example 5.
2TABLE 2 Median Content of Number of peaks not Comparative particle
particles not larger than median Electrode Electrode Example size
larger than 2 .mu.m particle size in particle density strength
Property No. (.mu.m) (Weight %) size distribution (g/cc)
(kgf/cm.sup.2) of sheet 1 6.3 5.3 0 0.623 3.8 Substantial cracks 2
5.9 7.8 0 0.631 4.4 Substantial cracks 3 6.2 6.9 0 0.630 3.9
Substantial cracks 4 7.1 4.0 0 0.620 3.1 Breakage of sheet 5 6.8
3.3 0 0.619 3.0 Breakage of sheet
[0110] FIG. 4 shows the particle size distribution of the activated
carbon obtained in Comparative Example 1. Alternating dotted dash
line in FIG. 4 indicates the median particle size in the particle
size distribution.
[0111] The amounts of benzene adsorbed by the activated carbon
obtained in Comparative Examples 1 to 5 were measured by the
measuring method specified in JIS standard K1474-1991 described
above, and it was confirmed that the amount of benzene adsorbed was
less than 54% or more than 60% by weight of benzene.
[0112] FIGS. 5A and 5B are graphs showing the relations between the
proportion of particles having median particle size (5B) r particle
sizes not larger than 2 .mu.m (5B) and density of the electrode
sheet.
[0113] From FIG. 5A, it was found that density of the electrode
sheet monotonically decreases as the median particle size of the
activated carbon increases. It was also found that density of the
electrode sheet is lower in the case of activated carbon which does
not have a peak observed on the side of smaller particle size than
the median particle size in the particle size distribution
(Comparative Example).
[0114] From FIG. 5B, it was found that density of the electrode
sheet monotonically increases as the content of particles not
larger than 2 .mu.m in the activated carbon increases.
[0115] FIGS. 6A and 6B are graphs showing the relationship between
the content of activated carbon particles not larger than 2 .mu.m
and the strength of the electrode sheet.
[0116] From FIG. 6A, it was found that very high strength of the
electrode sheet around 5 kgf/cm.sup.2 can be ensured by using
activated carbon that has a median particle size within a range
from 4 .mu.m to 8 .mu.m in the particle size distribution and at
least a peak observed on the side of smaller particle size than the
median particle size in the particle size distribution. It was
found that when the activated carbon that does not show a peak
observed on the side of smaller particle size than the median
particle size in the particle size distribution is used, the
strength of the electrode sheet decreases as the median particle
size increases even when median particle size falls within a range
from 4 .mu.m to 8 m in the particle size distribution.
[0117] From FIG. 6B, it can be seen that very high strength of the
electrode sheet around 5 kgf/cm.sup.2 can be ensured by using the
activated carbon that contains 10% or more particles having sizes
not larger than 2 .mu.m in terms of accumulated percentage.
Activated carbon of which the content of particles having sizes not
larger than 2 .mu.m in terms of accumulated percentage is less than
10% causes the strength of the electrode sheet to decrease and is
therefore undesirable.
[0118] It can be seen from the graph of FIG. 6 that very high
strength of the electrode sheet around 5 kgf/cm.sup.2 can also be
achieved by the activated carbon that has a median particle size
within a range from 4 .mu.m to 8 .mu.m in the particle size
distribution which enables it to make electrode sheet having a
relatively high density above 0.630 g/cm.sup.3 as shown in FIGS. 5A
and 5B, has at least a peak observed on the side of smaller
particle size than the median particle size in the particle size
distribution and contains 10% or more particles having sizes not
larger than 2 .mu.m in terms of accumulated percentage.
[0119] FIGS. 7A and 7B are graphs of accumulated particle size
distribution in Example 2 and Comparative Example 4 (7A) determined
from the results of measuring the particle size distribution of the
activated carbon in the Examples and Comparative Examples, while
FIG. 7B shows an enlarged view of a part of FIG. 7A.
[0120] From FIGS. 7A and 7B, it was confirmed that the activated
carbon obtained in Example 2 contains 10% or more particles having
sizes not larger than 2 .mu.m in terms of accumulated percentage,
while the activated carbon obtained in Comparative Example 4
contains particles having sizes not larger than 2 .mu.m in terms of
accumulated percentage by content of less than 10%.
[0121] From the results shown in FIGS. 5A and 5B, FIGS. 6A and 6B
and FIGS. 7A and 7B, it was confirmed that both relatively high
strength and density of electrode sheet can be achieved with
polarizing electrode for electric double layer capacitor made by
using activated carbon having median particle size within a range
from 4 .mu.m to 8 .mu.m, at least a peak observed on the side of
smaller particle size than the median particle size in the particle
size distribution and contains 10% or more particles having sizes
not larger than 2 .mu.m in terms of accumulated percentage.
[0122] Second Aspect of the Present Invention
[0123] The activated carbon that constitutes the polarizing
electrode for an electric double layer capacitor according to the
first aspect of the present invention is made by activating a
hard-to-graphitize material with water vapor.
[0124] The term hard-to-graphitize material is used for
comprehensive reference to materials made of organic compounds,
which are difficult to be graphitized. Difficulty in graphitizing
means that it is difficult to form a graphite structure by firing
at a temperature higher than 3000.degree. C. Formation of graphite
structure can be verified by observing a distinct peak of 2.theta.
around 25.degree. in the X-ray diffraction pattern.
[0125] The activated carbon that constitutes the polarizing
electrode for electric double layer capacitor of the second aspect
of the present invention is preferably made in a manufacturing
process based on the method described below.
[0126] First, stock feed used for manufacturing the activated
carbon of the present invention will be described. A
hard-to-graphitize material which is difficult to graphitize is
preferably used as the stock feed to manufacture the activated
carbon of the present invention. A graphitizing catalyst may be
added during graphitization. As the organic compounds which are
difficult to graphitize, furfuryl alcohol, polycarbonate,
cellulose, phenol resin and the like that are aromatic compounds,
and epoxy resin, PVDF (polyvinylidene fluoride), polyvinyl alcohol,
nylon and polypropylene that are aliphatic compounds may be
used.
[0127] The activated carbon that constitutes the polarizing
electrode for electric double layer capacitor of the present
invention can be manufactured in the following procedure by using
such materials as described above. A process of using phenol resin
as the hard-to-graphitize material, which is fired at such a
temperature as graphite structure is formed and is then activated
with water vapor will herein be described.
[0128] The heat treatment at such a temperature as graphite
structure is formed is carried out at a temperature usually within
a range from 400 to 1000.degree. C., preferably from 500 to
800.degree. C., and more preferably from 500 to 700.degree. C., in
a non-oxidizing atmosphere, for example, in the presence of
nitrogen (N.sub.2) gas flow. Duration of the treatment is usually
not longer than 24 hours, preferably from 1 to 10 hours, and more
preferably from 2 to 5 hours. Other conditions for the treatment
may be determined in accordance to such factors as the material to
be used and the kind of electrode to be made.
[0129] Activation with water vapor can be-carried out by an
ordinary method. In a preferred embodiment, activation with water
vapor is carried out as follows. A scrubbing bottle containing pure
water is kept at a temperature within a range from the room
temperature to 100.degree. C., preferably at 80.degree. C., and
nitrogen gas is passed through the water so that activation is
carried out by means of the nitrogen gas containing water vapor.
Specifically, temperature is raised to a level of 800 to
1000.degree. C., preferably 900.degree. C. in the presence of
nitrogen gas flow and, after a predetermined temperature (for
example, 800.degree. C.) has been reached, activation is carried
out by using a mixed gas of nitrogen and water vapor for a period
of 5 minutes to 10 hours.
[0130] The activated carbon prepared as described above is crushed
by a jet mill, ball mill or the like for 50 to 110 hours, and the
crushed powder is classified with a sieve of 330 mesh (45 .mu.m) so
as to obtain activated carbon having predetermined particle size
distribution for the polarizing electrode for electric double layer
capacitor of the present invention.
[0131] For the activated carbon prepared as described above,
activated carbon that contains 10% or more particles having sizes
not larger than 2 .mu.m in terms of accumulated percentage, a bulk
density within a range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3, and
a fluidity index within a range from 0.47 to 0.52 as calculated by
the Kawakita's formula from tap density can be made.
[0132] It was confirmed, by using the measuring method specified in
JIS standard K1474-1991, that the amount of benzene adsorbed by the
activated carbon thus obtained was not less than 54% and not more
than 60% by weight of benzene.
[0133] The amount of benzene adsorption is an index that represents
the degree of the progress of activation which is determined by
measuring the weight difference of benzene vapor adsorbed onto the
activated carbon.
[0134] The activated carbon can then be used to make the polarizing
electrode for an electric double layer capacitor by an ordinary
method. In order to make a sheet-like polarizing electrode, for
example, the following method may be used.
[0135] The activated carbon made from phenol resin described above,
graphite powder used as an electrically conductive filler and
ethylene polytetrafluoride used as a binder are mixed in
predetermined proportions (for example, 90:5:5 in weight
proportion) and rolled into a sheet 150 .mu.m in thickness. The
sheet is punched through in a circular shape to make a polarizing
electrode having a diameter of 20 mm.
[0136] Then, as shown in FIG. 8, an electrode unit made by
sandwiching a separator with two sheet-like polarizing electrodes 2
and 3 is disposed in a casing 6 made of an electrically conductive
material that serves as the cathode. After pouring an electrolytic
solution into the casing, a cap 5 made of an electrically
conductive material that serves as the anode is placed thereon and
edges of the casing 6 and the cap 5 are calked together via a
packing 9 made of an insulating material for sealing, thereby to
make the electric double layer capacitor 1.
[0137] The content of fine powder of the activated carbon not
larger than 2 .mu.m in particle size which is an index that
represents the activated carbon can be determined by the measuring
method using SALD-3000S laser particle size analyzer manufactured
by Shimadzu Corporation. The bulk density can be determined by the
bulk density measuring method for particulate activated carbon
specified in JIS K1474.
[0138] The fluidity index that represents the property of activated
carbon can be determined by Kawakita's formula using Kawakita's tap
density measuring method (weight tap density measuring method using
Tap Denser KYT-3000 manufactured by SEISHIN ENTERPRISE CO., LTD.).
With the weight tap density measuring method, tap density is
measured by filling a 100 cc cylinder with about 20 g of activated
carbon passed through a sieve having a mesh size of 710 .mu.m.
Volume loss ratio C=(V.sub.0-V.sub.N)/V.sub.0 of the activated
carbon is calculated from the number of tapping cycles N, initial
volume of the activated carbon V.sub.0 and volume of the activated
carbon V.sub.N after N cycles of tapping. The relationship between
N and N/C (N/C=xN+y) is derived from a graph drawn by plotting N/C
along the vertical axis and N along the horizontal axis, so as to
calculate the fluidity index by Kawakita's formula, thereby to
evaluate the fluidity.
[0139] Kawakita's formula is expressed as N/C=(1/a)N+1/(ab), where
a is the fluidity index (a number that represents fluidity), b is
the inverse number of adhesion index (a number that represents the
magnitude of adhesive force) 1/b. Thus the fluidity index a can be
determined as the inverse number of x in N/C=xN+y that is derived
from the relation of N and N/C.
[0140] When the fluidity index of the activated carbon increases
indicating the fluidity of the powder becoming higher, stress
caused by the pressure of rolling applied for the purpose of
increasing the density tends to be redirected from the direction of
pressing down to the direction of stretching the sheet which is at
right angles to the direction of pressing down in the rolling
process during manufacture of the electrode sheet. As a result,
cracks are likely to occur due to decreasing density., decreasing
thickness and stretching of the electrode sheet.
[0141] The strength of the electrode sheet which is an index that
represents the performance of the capacitor can be determined by
measuring the tensile strength by using, for example, EZ Test-100N
manufactured by Shimadzu Corporation. The density of the electrode
sheet can be determined, for example, by measuring the apparent
density using a micrometer.
[0142] The ratio of capacity maintained after endurance is the
electrostatic capacity demonstrated after 2000 hours of 2.5 V
continuous voltage application test conducted at 45.degree. C.
divided by the electrostatic capacity demonstrated before applying
the voltage, given in percentage. The electrostatic capacity can be
determined from the energy discharged by repeating charge and
discharge with predetermined voltage and current (for example,
charge voltage of 2.5 V and charge current of 5 mA) using a
predetermined electrolytic solution (for example, propylene
carbonate solution of triethylmethylammonium tetrafluoroborate:
TEMA.cndot.BF.sub.4/PC, 1.8 mol/l in concentration).
Examples
[0143] The present invention will be described below by way of
examples, but it should be noted that the present invention is not
limited by the following examples.
Example 7
[0144] The activated carbon powder of this example was made by the
following procedure.
[0145] (1) A phenol resin granulated to have a particle size of
about 3 mm was carbonized by maintaining in a nitrogen gas flow at
900.degree. C. for two hours.
[0146] (2) Carbon thus obtained was heated again in the nitrogen
gas flow and, when the temperature reached 800.degree. C., the
nitrogen gas containing 5% of water vapor and 5% of carbon dioxide
was supplied and the carbon was kept at 900.degree. C. (hereinafter
referred to as an activation temperature) for one hour (hereinafter
referred to as the activation time), so as to be activated.
[0147] (3) The activated carbon thus obtained was left to cool down
and was crushed by using a ball mill employing high-purity alumina
balls and operated with rotational speed of 15 rpm for 90 hours
(hereinafter referred to as the crushing time), thereby to obtain
the activated carbon of this example.
[0148] The content of particles not larger than 2 .mu.m in the
activated carbon made as described above was determined by the
laser diffraction method (using the SALD-3000S manufactured by
Shimadzu Corporation)
[0149] The bulk density was determined by the bulk density
measuring method for particulate activated carbon specified in JIS
K1474.
[0150] The fluidity index was determined by Kawakita's formula
using a Kawakita's tap density measuring method (measuring method
using Tap Denser KYT-3000 manufactured by SEISHIN ENTERPRISE CO.,
LTD.).
[0151] The activated carbon was mixed with 5% by weight of Teflon
7J.RTM. (manufactured by Du Pont-Mitsui Fluorochemicals Co., LTD.)
and 5% of Denka Black.RTM. (manufactured by DENKI KAGAKU KOGYO
KABUSHIKI KAISHA) added thereto as binder, and was formed by
pressure powder molding into a polarizing electrode measuring 20 mm
in diameter and 150.mu.m in thickness measured with a micrometer.
The polarizing electrode was dried at 150.degree. C. under vacuum
for four hours, and was weighed to determine the density of the
electrode.
[0152] The strength of the electrode was determined by measuring
the tensile strength of the electrode in the condition described
above by using EZ Test-100N manufactured by Shimadzu
Corporation.
[0153] Table 3 shows the results of measuring the activated carbon
obtained in Example 7 by a Kawakita's tap density measuring method
(number of tapping cycles N, volume of the activated carbon V.sub.N
after N cycles of tapping and volume loss ratio
C=(V.sub.0-V.sub.N)/V.sub.0 of the activated carbon (V.sub.0 is the
initial volume of activated carbon, and is shown differently in
Table 1 by assuming VO to be 100). FIG. 10 shows the results of
deriving the relationship between N and N/C (N/C=xN+y) from a graph
drawn by plotting N/C along the vertical axis and N along the
horizontal axis.
3 TABLE 3 N V C N/C 5 95.00 0.05 100.00 10 90.00 0.10 100.00 15
87.00 0.13 115.38 20 83.00 0.17 117.65 50 72.00 0.28 178.57 100
66.00 0.34 294.12 300 59.00 0.41 731.71 500 58.00 0.42 1190.48 1000
56.00 0.44 2272.73 1500 54.00 0.46 3260.87 2000 53.00 0.47 4301.08
2500 53.00 0.47 5319.15
[0154] Table 4 shows the median particle size of the activated
carbon obtained in Example 1, bulk density, fluidity index, density
of the polarizing electrode, strength of the electrode and property
(moldability) of the electrode sheet.
[0155] A PC solution of
1.8M(C.sub.2H.sub.5).sub.3CH.sub.3N.BF.sub.4 was used as the
electrolic solution, and electrostatic capacity was determined from
the energy discharged by charging for two hours with constant
current and constant voltage with charge voltage 2.5 V and charge
current of 5 mA. The ratio of capacity deterioration after
endurance was determined by dividing the electrostatic capacity
demonstrated after 2000 hours of 2.5 V continuous voltage
application test conducted at 45.degree. C. by the electrostatic
capacity demonstrated before applying the voltage, given in
percentage.
[0156] The result showed that the ratio of capacity maintained
after 2000 hours was 90%.
Examples 8 to 12
[0157] In these examples, activated carbon was made in the same
manner as in Example 7, except for setting the crushing time to 95
hours (Example 8), activation time to 2 hours and crushing time to
90 hours (Example 9), activation time to 2 hours and crushing time
to 95 hours (Example 10), activation time to 2.5 hours and crushing
time to 90 hours (Example 11), and activation time to 2.5 hours and
crushing time to 95 hours (Example 12). The median particle size of
the activated carbon, number of peaks observed on the side of
smaller particle size than the median particle size in the particle
size distribution, content of particles not larger than 2 .mu.m,
the density of the polarizing electrode, the strength of the
electrode and property (moldability) of the electrode sheet were
also determined by similar methods. These figures are also shown in
Table 4.
[0158] The ratio of capacity maintained after 2000 hours was 91%
for the electrode sheet made by using the activated carbon obtained
in Example 8, ratio of capacity maintained after 2000 hours was 92%
in the case of Example 9, ratio of capacity maintained after 2000
hours was 92% in the case of Example 10, ratio of capacity
maintained after 2000 hours was 93% in the case of Example 11, and
ratio of capacity maintained after 2000 hours was 94% in the case
of Example 12.
4TABLE 4 Median particle Bulk Electrode Electrode Proper- Example
size density Fluidity density strength ty No. (.mu.m) (g/cc) index
(g/cc) (kgf/cm.sup.2) of sheet 7 5.8 0.250 0.470 0.662 5.1 Good 8
5.9 0.235 0.482 0.656 5.2 Good 9 6.4 0.212 0.493 0.650 5.1 Good 10
6.1 0.201 0.484 0.651 5.3 Good 11 6.3 0.195 0.520 0.643 5.0 Good 12
6.4 0.190 0.509 0.645 4.9 Good
[0159] Particle size distributions of the activated carbons
obtained in Examples 7 to 12 were determined by the laser
diffraction method (using SALD-3000S manufactured by Shimadzu
Corporation), and it was confirmed that the activated carbons have
median particle size within a range from 4 .mu.m to 8 .mu.m in the
particle size distribution, and have at least a peak observed on
the side of smaller particle size than the median particle size in
the particle size distribution.
[0160] It was confirmed, by using the measuring method specified in
JIS standard K1474-1991, that the amounts of benzene adsorbed by
the activated carbons obtained in Examples 7 to 12 were not less
than 54% and not more than 60% by weight of benzene.
Comparative Example 6
[0161] In this example, activated carbon was made in the same
manner as in Example 7, except for setting the activation time to 4
hours, the rotational speed of the crusher to 15 rpm and crushing
time to 100 hours. The median particle size of the activated
carbon, number of peaks observed on the side of smaller particle
size than the median particle size in the particle size
distribution, content of activated carbon particles not larger than
2 .mu.m, density of the polarizing electrode, the strength of the
electrode and property (moldability) of the electrode sheet were
also determined by similar methods. These figures are shown in
Table 5.
[0162] The ratio of capacity maintained after 2000 hours was 86%
for the electrode sheet made by using the activated carbon obtained
in Comparative Example 6.
Comparative Examples 7 to 10
[0163] In these examples, activated carbon was made in the same
manner as in Comparative Example 6, except for setting the crushing
time to 110 hours (Comparative Example 7), activation time to 4.5
hours and crushing time to 110 hours (Comparative Example 8),
activation time to 0.5 hours, rotational speed of the crusher to 25
rpm and crushing time to 50 hours (Comparative Example 9), and
activation time to 0.5 hours, rotational speed of the crusher to 25
rpm and crushing time to 60 hours (Comparative Example 10). The
median particle size of the activated carbon, number of peaks
observed on the side of smaller particle size than the median
particle size in the particle size distribution, content of
particles not larger than 2 .mu.m, density of the polarizing
electrode, the strength of the electrode and property (moldability)
of the electrode sheet were also determined by similar methods.
These figures are also shown in Table 5.
5TABLE 5 Compar- Median ative particle Bulk Flu- Electrode
Electrode Example size density idity density strength Property No.
(.mu.m) (g/cc) index (g/cc) (kgf/cm.sup.2) of sheet 6 7.0 0.178
0.548 0.611 4.3 Breakage of sheet 7 6.9 0.179 0.567 0.610 3.7
Breakage of sheet 8 6.2 0.169 0.573 0.608 3.4 Breakage of sheet 9
6.4 0.262 0.461 0.689 4.7 Breakage of sheet 10 5.3 0.270 0.458
0.691 3.8 Breakage of sheet
[0164] The ratio of capacity maintained after 2000 hours was 83%
for the electrode sheet made by using the activated carbon obtained
in Comparative Example 7, ratio of capacity maintained after 2000
hours was 87% in the case of Comparative Example 8, ratio of
capacity maintained after 2000 hours was 76% in the case of
Comparative Example 9, and ratio of capacity maintained after 2000
hours was 73% in the case of Comparative Example 10.
[0165] Particle size distributions of the activated carbons
obtained in Comparative Examples 6 to 10 were determined by the
laser diffraction method (using SALD-3000S manufactured by Shimadzu
Corporation), and it was confirmed that median particle sizes of
the activated carbons were less than 4 .mu.m or larger than 8 .mu.m
in the particle size distribution and the particle size
distributions do not have peak observed on the side of smaller
particle size than the median particle size.
[0166] It was confirmed, by using the measuring method specified in
JIS standard K1474-1991 described above, that the amounts of
benzene adsorbed by the activated carbons obtained in Comparative
Examples 6 to 10 were less than 54% or more than 60% by weight of
benzene.
[0167] FIGS. 11A and 11B are graphs showing the relationship
between the bulk density (11A) or fluidity index (11B) of the
activated carbon and density of the electrode sheet.
[0168] From FIG. 11A, it was found that the activated carbon having
bulk density within a range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3
makes electrode sheet of which density increases at relatively high
levels above 0.630 g/cm.sup.3 as the bulk density increases within
this range.
[0169] From FIG. 11B, it was found that activated carbon having a
fluidity index within a range from 0.47 to 0.52 makes an electrode
sheet of which density decreases at relatively high levels above
0.630 g/cm.sup.3 as the fluidity index increases within this
range.
[0170] FIGS. 12A and 12B are graphs showing the relationship
between the bulk density (12A) or fluidity index (12B) of activated
carbon and the strength of the electrode sheet.
[0171] From FIG. 12A, it was found that the activated carbon that
has a bulk density within a range from 0.18 g/cm.sup.3 to 0.25
g/cm.sup.3 enables it to ensure a very high strength of the
electrode sheet of around 5 kgf/cm.sup.2.
[0172] From FIG. 12B, it was found that the activated carbon that
has a fluidity index within a range from 0.47 to 0.52 enables it to
ensure a very high strength of the electrode sheet of around 5
kgf/cm.sup.2.
[0173] It can be seen from the graphs of FIGS. 12A and 12B that a
very high strength of the electrode sheet around 5 kgf/cm.sup.2 can
also be achieved by the activated carbon that has a bulk density
within a range from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3 which
enables it to make electrode sheet having a relatively high density
above 0.630 g/cm.sup.3 and fluidity index within a range from 0.47
to 0.52 as shown in FIGS. 11A and 11B.
[0174] FIGS. 13A and 13B are graphs of accumulated percentage of
particle size distribution in Example 8 and Comparative Example 9
determined from the results of particle size distribution of the
activated carbon in the Examples and Comparative Examples, while
FIG. 13B shows an enlarged view of a part of FIG. 13A.
[0175] Form FIGS. 13A and 13B, it was confirmed that the activated
carbon obtained in Example 8 contains 10% or more particles having
sizes not larger than 2 .mu.m in terms of accumulated percentage,
while the activated carbon obtained in Comparative Example 9
contains less than 10% of particles having sizes not larger than 2
.mu.m in terms of accumulated percentage.
[0176] From the results shown in FIGS. 11A and 11B, FIGS. 12A and
12B and FIG. 13A and 13B, it was confirmed that both relatively
high strength and density of the electrode can be achieved with
polarizing electrode for an electric double layer capacitor made by
using the activated carbon that has a bulk density within a range
from 0.18 g/cm.sup.3 to 0.25 g/cm.sup.3 and a fluidity index within
a range from 0.47 to 0.52.
[0177] While preferred aspects of the present invention have been
described and illustrated above, it should be understood that these
are exemplary of the invention and are not to be considered as
limiting. Additions, omissions, substitutions, and other
modifications can be made without departing from the spirit or
scope of the present invention. Accordingly, the invention is not
to be considered as limited by the foregoing description and is
only limited by the scope of the appended claims.
* * * * *